It is well-known to transfer energy from a source such as a motor or internal combustion engine to a load through the intermediary of hydraulic drive system. Such systems will typically have a pump driven by the source and a motor connected to the load. By adjusting the hydraulic flow between the pump and the motor it is possible to impart movement to the load, maintain it in a fixed position and otherwise influence its disposition.
The control of fluid flow is typically accomplished by a valve mechanism, which in its simplest form simply opens or closes the flow between the pump and motor and thereby regulates movement of the load. Such valve systems are relatively inefficient in terms of the energy dissipated across the valve. In a typical installation, the valve would be closed centered requiring the pump to deliver pressure against a relief valve. The energy provided to the fluid is thus dissipated as heat. In an open center arrangement, careful manufacture of the valve is required in order to obtain the transition between the zero flow and full flow whilst retaining control of the load and metering of the flow across the valve, which causes loss of energy.
The valves used to control flow therefore are relatively complicated and made to a high degree of precision in order to attain the necessary control function. As such, the valves tend to be specialized and do not offer flexibility in implementing different control strategies. Most significantly, since the control is achieved by metering flow across an orifice there is inherently significant energy loss when controlling fluid flow. The control valve regulates movement by controlling flow across a restricted port at the inlet to the device. Because the control valve is typically a one piece spool, a similar restricted port is presented to the exhaust flow and results in a significant energy loss.
What is desired therefore, is an improved system for unloading a hydraulic pump that can eliminate energy consumption of the hydraulic pump when work is not being performed and extend the hydraulic pump life. What is also desired is an unloading system that provides a power density and packaging envelope advantage by saving space through the integration of components. What is further desired is an unloading system that experiences substantially less parasitic loss and reduces environmental noise emission over traditional hydraulic bypass systems, thus allowing for ease of use, and increased versatility.